: Neurons maintain their axons by constantly supplying them with macromolecules synthesized in the cell body. This supply is sustained by mechanisms of axonal transport, which move different groups of macromolecules to the distal axon at different velocities. In neurological diseases, this transport could fail either as a primary cause of the disease or as a consequence induced by other primary defects. In either case neuronal degeneration would be exacerbated. Ample evidence suggest that defective axonal transport exists in motor neuron disease, leading to the accumulation of neurofilaments and other organelles in the proximal axons and cell bodies. The proposed study takes advantage of existing transgenic mice to test three specific hypotheses regarding axonal transport. The long term objective is to further understand mechanisms of axonal transport in both normal and pathological states. The first hypothesis is that NF-H slows axonal transport of NFs and other cytoskeletal components by stabilizing the cytoskeletal network and that the normal rate of axonal transport of NFs is determined by a balanced ratio of the three NF subunits. To test this hypothesis, the rate of slow axonal transport will be measured in a variety of transgenic mice that express elevated levels of different NF subunits. The second hypothesis is that the axonal cytoskeletal network hinders the movement of organelles, and thus, plays a significant role in determining the rates of transport of different organelles. To test this hypothesis, the fast axonal transport rates of different groups of proteins (associated with different organelles) will be measured in mice that have elevated NF densities and crossbridges, Finally, the third hypothesis is that accumulation of NFs and membranous organelles in motor neuron disease is caused by defects in axonal transport. To test this hypothesis, the rate of both fast and slow axonal transport will be measured in mice that develop motor neuron disease due to expression of a mutant Cu/Zn superoxide dismutase (SOD1). The axonal transport rates will be measured using a classical paradigm in which a radioactive amino acid (35S-methionine) is delivered to the immediate vicinity of neuronal cell bodies. As the amino acid is incorporated into newly synthesized proteins, the transport of these proteins along the axons will be monitored at different time points.